Bull gear of a differential gear assembly

ABSTRACT

A lower cost differential gear assembly capable of inputting driving force through a differential gear, whose teeth have sufficient strength is provided. To achieve the object, the differential gear is constructed by engaging a gear part provided with teeth at equal intervals on one of its perimeter surfaces, and a support part pivotally supported by right and left axles through engagement of a spline. Accordingly, the gear part can be formed from a high strength material and the support part can be formed from a lower cost material.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Continuation of application Ser. No.10/725,598, filed Dec. 3, 2003, which is incorporated in its entiretyherein by reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bull gear of a differential gearassembly (a differential bull gear).

2. Related Art

Conventionally, a differential bull gear is made of high-strength metalsuch as steel or powder metal sintered in high density because its teethespecially require high strength. However, the bull gear entirely madeof such high-strength material is expensive.

Therefore, as disclosed in U.S. Pat. Nos. 6,338,690 and 2,608,261, forexample, a well-known conventional economical differential bull gearcomprises integrally coupled different outer and inner members, whereinan outer member serving as a ring gear is made of high-strengthmaterial, and an inner member, a plate which supports a differentialpinion, does not require such high strength and may be made ofinexpensive material.

However, in this type of bull gear, a problem arises because the twodifferent members coupled together may axially shift from each other soas to hinder proper movement of the differential gear assembly.

The differential bull gear disclosed in U.S. Pat. No. 6,338,690 avoidsthe axially positional difference between the inner and outer members byinserting corners of the square thin inner member into correspondingrecesses formed in the inner peripheral portion of the outer memberserving as a ring gear. However, the thin inner member supporting just adifferential pinion is so separated from a pair of axles as to make itdifficult to fix the axles in axial location when differential sidegears fixed on the respective axles mesh with the differential pinion.

Therefore, a differential housing is divided into halves along the bullgear so that the differential housing halves sandwich the bull gear.Each of the differential housing halves is partly extended to fill gapsbetween the square inner plate and the ring gear so as to reinforce thebull gear and enhance efficiency of torque transmission between the ringgear and the inner plate. However, each of the differential housinghalves having such a complicated shape and requiring high-dimensionalaccuracy may be expensive, and the differential gear assembly iscomplicated in its assembly.

In the differential bull gear disclosed in the U.S. Pat. No. 2,608,261,the inner member supporting the differential pinion and supported onaxles has a circular contour, when axially viewed, so that theperipheral surface thereof entirely contacts a inner peripheral surfaceof the outer member serving as a ring gear. Both members are preventedfrom relative rotation by tightening some screws, and both contact headsof the screws so as to be prevented from axially relative shifting.

However, each of female screws for the screws is divided into halves.The female screw halves are formed on the inner peripheral surface ofthe outer member and the outer peripheral member of the inner member,respectively. The inner and outer members should be accurately locatedin their relatively rotational and axial directions so as to form thefemale screws. Since the half-divided female screws require complicatedprocessing and location, and high accuracy, the resultant bull gear maybe expensive. If the bull gear is simplified by decreasing the screwsand female screws, efficiency of torque transmission between the innerand outer members is reduced, and the screws are further stressed so asto be damaged, causing rotational and axial moving of the inner andouter members.

BRIEF SUMMARY OF THE INVENTION

A main object of the present invention is to provide an economical bullgear of a differential gear assembly (a differential bull gear)simplified in assembly and location while keeping sufficient strength ofits toothed portion as an input gear for receiving torque from atransmission.

To achieve the object, a differential bull gear according to the presentinvention comprises a ring gear for inputting torque from atransmission, and a support member supporting a differential pinion andsupported on an axle. The support member transmits the torque from thering gear to the axle through the differential pinion. The ring gear andthe support member are toothed to be coupled together so as to berelatively axially shiftable and not-relatively rotatable. The ring gearmade of high-strength material such as steel or sintered powder metal issufficiently strong to act as the input gear for receiving torque from atransmission. The support member allowed to be lower in strength thanthe ring gear is made of sintered powder metal, plastics, die-castmetal, or the like, thereby being inexpensive.

In one aspect, the ring gear preferably has a toothed outer peripheryand a toothed inner periphery. One of the toothed outer and innerperipheries serves as an input gear for receiving torque from thetransmission. The other of the toothed outer or inner periphery mesheswith the toothed portion of the support member so as to couple the ringgear with the support member.

Alternatively, in another aspect, the ring gear preferably has a toothedouter periphery and a toothed inner periphery. Either the toothed outeror inner periphery serves as an input gear for receiving torque from thetransmission. Both toothed outer and inner peripheries mesh with thetoothed portion of the support member so as to couple the ring gear withthe support member.

In this aspect, the support members has an annular recess into which thering gear is fitted. The recess has a toothed outer periphery and atoothed inner periphery for meshing with the toothed outer and innerperipheries of the ring gear, respectively.

A second object of the present invention is to provide the bull gearwith the support member and the ring gear prevented from relativelyaxially moving by simple configuration.

As an aspect, the above-mentioned support member, is preferablydivisible into halves, each half having an annular recess. The halvesare joined to each other so as to fit the ring gear in the mutuallyfacing recesses, thereby forming the support member holding the ringgear.

As an aspect, while the ring gear has a first surface perpendicular tothe axle and the support member has a second surface to be leveled withthe first surface of the ring gear, a retaining member abuts against thefirst and second surfaces so as to prevent the ring gear and the supportmember from relatively axial moving. The retaining member may beprovided on a pinion shaft supporting a pinion for transmitting torquefrom the transmission to the ring gear. Alternatively, the retainingmember may be a washer provided on a screw screwed into either the ringgear or the support member. If the outer member is divided into aplurality of pieces, a plurality of the retaining members may beprovided to the respective pieces of the outer member.

These, other and further objects, features and advantages of theinvention will appear more fully from the following description taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

FIG. 1 is a side view of a mower tractor equipped with a transaxle.

FIG. 2 is a sectional plan view of the transaxle incorporating adifferential gear assembly including a differential bull gear accordingto a first embodiment of the present invention.

FIG. 3 is a sectional rear view of the transaxle.

FIG. 4 is a side view of the differential bull gear according to thefirst embodiment.

FIG. 5 is a side view of a differential bull gear according to a secondembodiment of the present invention.

FIG. 6 is a side view of a differential bull gear according to a thirdembodiment of the present invention.

FIG. 7 is a sectional plan view of a differential gear assemblyincluding the differential bull gear according to the third embodiment.

FIG. 8 is a side view of a differential bull gear according to a fourthembodiment of the present invention.

FIG. 9 is a sectional plan view of a differential gear assemblyincluding the differential bull gear according to the fourth embodiment.

FIG. 10 is a side view of a differential bull gear according to a fifthembodiment of the present invention.

FIG. 11 is a sectional plan view of a differential gear assemblyincluding the differential bull gear of the fifth embodiment.

FIG. 12 is a side view of a differential bull gear according to a sixthembodiment of the present invention.

FIG. 13 is a side view of a differential bull gear according to aseventh embodiment of the present invention.

FIG. 14 is a sectional plan view of a differential gear assemblyincluding the differential bull gear according to the seventhembodiment.

FIG. 15 is a perspective view of a center section for a hydraulicstepless transmission in the transaxle shown in FIGS. 2 and 3.

FIG. 16 is a perspective view of a movable swash plate for the hydraulicstepless transmission in the transaxle shown in FIGS. 2 and 3.

FIGS. 17 to 23 are sectional side views of some types of breathers forthe transaxle shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

As an example of a vehicle equipped with a differential gear assembly,as shown in FIG. 1, a mower tractor 10 is provided at its rear portionwith a transaxle 1, incorporating a differential gear assemblydifferentially supporting right and left axles 35. Right and left rearwheels 16 are fixed onto outer ends of the axles 35, and right and leftfront wheels 15 are suspended at a front portion of the mower tractor10. An engine 11 is mounted above the front wheels 15. The engine 11 hasa downwardly vertical output shaft 12 with a pulley 13 fixed thereon.The transaxle 1 has an upwardly vertical input shaft 24 with a pulley 23fixed thereon.

A drive belt 17 is wound around the pulleys 13 and 23 so as to transmitdriving force from the engine 11 to the input shaft 24 of the transaxle1. As shown in FIGS. 2 and 3, the transaxle 1 incorporates a hydraulicstepless transmission (hereafter, a HST) 25, a reduction gear assembly81 and a differential gear assembly 82. The driving force of the inputshaft 24 is transmitted to the right and left axles 35 through the HST25, the reduction gear assembly 81, and the differential gear assembly82, thereby rotating the rear wheels 16.

The mower tractor 10 is provided with an accelerator 26 comprising afront pedal 26 a and a rear pedal 26 b, which are connected integrallyso as to be rotatable like a seesaw centered on a horizontal pivot shaft27. The mower tractor 10 travels forward by pressing the pedal 26 a andtravels rearward by pressing the pedal 26 b.

An arm 28 is fixed to the accelerator 26 so as to be rotatable aroundthe pivot shaft 27. The arm 28 is pivotally connected to a speed changelever 30 through a rod 29. The speed change lever 30 is fixed onto ahorizontal operation shaft 31 extended outward from the HST 25.Therefore, depression of either the front or rear pedal 26 a or 26 b istransmitted to the operation shaft 31 of the HST 25.

A mower deck 22 incorporating a rotary blade 21 for cutting grass isdisposed below the center portion of the vehicle body. A pulley 14 isalso fixed on the output shaft 12 of the engine 11, and a pulley 19 isfixed on an input shaft 20 extended upward from the mower deck 22. Adrive belt 18 is wound around the pulleys 14 and 19 so as to transferdriving force from the engine 11 into the mower deck 22, therebyrotating the rotary blade 21.

The transaxle 1 will be explained with reference to FIGS. 2 and 3. Thetransaxle 1 comprises a housing 33, which is dividable into an upperhousing member 33 a and a lower housing member 33 b. The housing 33incorporates the HST 25, a brake assembly 80, the reduction gearassembly 81 and the differential gear assembly 82. The HST 25 comprisesa hydraulic motor 36, a hydraulic pump 37, a center section 38 andothers.

The vertical input shaft 24 is extended above the housing 33. The pulley23 and a cooling fan 32 are fixed on the upward extended portion of theinput shaft 24 above the housing 33.

The input shaft 24 serves as a pump shaft of the hydraulic pump 37 sothat, by rotating the input shaft 24, the hydraulic pump 37 supplieshydraulic oil to the hydraulic motor 36.

The variable displacement hydraulic pump 37 comprises a cylinder block44 and a movable swash plate 45. In the cylinder block 44 is disposedthe axial pump shaft 24 and vertically slidable pistons 43. The movableswash plate 45 is integrally provided with a thrust bearing 46 abuttingagainst the pistons 43. The movable swash plate 45 has an arcuatelycurved surface 45 a (see FIG. 16) slidably fitted to the housing 33.

In the housing 30, a connection arm 50 with a horizontal projection 50 ainserted into the movable swash plate 45 is fixed on the operation shaft31. By rotating the speed change lever 30, the operation shaft 31 andthe connection arm 50 are rotated integrally with the lever 30, therebychanging a tilt angle (and direction) of the movable swash plate 45 soas to change the amount and direction of oil discharged from thehydraulic pump 37 to the hydraulic motor 36.

An adjuster 51 for adjusting the neutral position of the connection arm50 is attached to the housing 33 so as to correct the neutral positionof the movable swash plate 45.

The center section 38 is fastened to the housing 33 by bolts so as to beapart from the bottom of the housing 33. An oil filter 47 is disposedbetween the center section 38 and the bottom of the housing 33. Thecenter section 38 having a horizontal pump mounting surface 38 b and avertical motor mounting surface 38 a (see FIG. 15) is arranged in thehousing 33 so as to place the pump mounting surface 38 b between theaxle 35 and the motor mounting surface 38 a. The vertically axialcylinder block 44 of the hydraulic pump 37 is slidably and rotatablyfitted onto the pump mounting surface 38 b, and a horizontally axialcylinder block 40 of the hydraulic motor 36 is slidably and rotatablyfitted onto the motor mounting surface 38 a.

The fixed displacement hydraulic motor 36 comprises the cylinder block40 with horizontal pistons 39 inserted therein, and a fixed swash plate41. A thrust bearing 42 is integrally provided in the swash plate 41 soas to abut against the pistons 39.

The hydraulic motor 36 has a horizontal motor shaft 74 disposed parallelto the axles 35. The motor shaft 74 axially and not-relatively rotatablypenetrates the cylinder block 40. The motor shaft 74 is extended fromthe cylinder block 40 and freely passed through the swash plate 41, anda gear 75 is fixedly provided thereon. Accordingly, output from thehydraulic motor 36 is transmitted to the gear 75. The brake assembly 80is provided around the gear 75 so as to apply braking force onto themotor shaft 74.

A counter shaft 79 is supported between an outer wall and a partition ofthe housing 33. An axially long and diametrically small pinion 77rotatably fitted on the shaft 79, and a diametrically large gear 76not-relatively rotatably fitted on the pinion 77 constitute thereduction gear assembly 81. The gear 76 meshes with the gear 75, and thepinion 77 meshes with a differential bull gear 78 of the differentialgear assembly 82. Thus, the reduction gear assembly 81 transmits outputof the hydraulic motor 36 to the differential gear assembly 82.

The differential gear assembly 82 differentially rotates the right andleft axles 35. The right and left axles 35 are rotatably supported bythe rear portion of the housing 33, and extended outward therefrom. Therear wheels 16 are fixed onto distal ends of the axles 35.

As described above, output of the engine 11 is transmitted to the rightand left axles 35 through the HST 25, the reduction gear assembly 81,and the differential gear assembly 82, thereby rotating the rear wheels16.

As shown in FIG. 3, in the housing 33, a vacant space above the gear onthe counter shaft 79 serves as an oil tank 100. A breather 101 isdisposed on an upper surface of the housing 33 above the oil tank 100,as described below.

The differential gear assembly 82 will now be explained. As shown inFIG. 2, bevel side gears 34 are fixed on proximal portions of the rightand left coaxial axles 35. The bull gear 78 disposed between the bevelside gears 34 meshes with the pinion 77 so as to serve as an input gearof the differential gear assembly 82. The bull gear 78 has an axialcenter hole 91 a into which proximal ends of the axles 35 are relativelyrotatably inserted so as to support the bull gear 78.

At least one differential bevel pinion 93 is rotatably supported in thebull gear 78 through a pinion shaft 92, and meshes with the right andleft side gears 34 so as to differentially transmit torque of the bullgear 78 to the axles 35.

The differential bull gear 78 will be explained. As shown in FIGS. 4 and5, the differential bull gear 78 comprises an outer ring gear part 90and an inner support part 91. The ring gear part 90 has an outerperipheral gear to mesh with the pinion 77, and also has an innerperipheral gear. The support part 91 has an outer peripheral gear, whichmeshes with the inner peripheral gear of the ring gear part 90, so thatthe ring gear part 90 and the support part 91 are not-relativelyrotatably but they are relatively axially slidably joined with eachother. In other words, the ring gear part 90 and the support part 91 arerelatively axially slidable so as to be separated from each other.

The axial center hole 91 a into which the proximal ends of the axles 35are inserted is formed in the support part 91. The support part 91 alsohas at least one hole 91 b in which the pinion shaft 92 is supported andthe pinion 93 on the shaft 92 is allowed to rotate.

The support part 91 of the bull gear 78 with two pinions 93 as a firstembodiment shown in FIG. 4 is provided with two holes 91 b for therespective pinions 93 and shafts 92. The two holes 91 b, and the shafts92 with pinions 93 therein are symmetrical with respect to the axialcenter hole 91 a (the axles 35). The support part 91 of the bull gear 78with three pinions 93 as a second embodiment shown in FIG. 5 is providedwith three holes 91 b for the respective pinions 93 and shafts 92. Thethree holes 91 b, and the shafts 92 with the pinions 93 therein arearranged regular-triangularly around the axial center hole 91 a (theaxles 35).

As shown in FIGS. 4 and 5, vacant holes 91 c for weight-reduction may besuitably provided in the support part 91 in addition to the axial centerhole 91 a and the holes 91 b.

As shown in FIG. 2, on the counter shaft 79 are axially-immovably fittedright and left radial retaining rings 94 through the pinion 77 so as toabut against right and left side surfaces of the ring gear part 90 andthe support part 91, respectively. The right and left side surfaces ofthe ring gear part 90 are leveled with the right and left side surfacesof the support part 91, respectively, so as to surely abut against therespective retaining rings 94. Therefore, the right and left retainingrings 94 sandwich the bull gear 78 while preventing the ring gear part90 and the support part 91 from relatively axially shifting.

The ring gear part 90 is made from high-strength metal material, such asforged steel or high-density sintered powder metal. The support part 91may be made from economical material, such as plastics, die-cast metal,or low-density sintered powder metal, having lower strength than thering gear part 90. Accordingly, the differential bull gear 78 may bereduced in cost while keeping the required strength for its teeth toperform as an input gear receiving torque from the reduction gearassembly 81. Further, the ring gear part 90 and the support part 91 arejoined together through their meshing teeth so as to ensure sufficienttransmission of torque therebetween.

A third embodiment of the differential bull gear 78 shown in FIGS. 6 and7 will be explained. While the inner support part 91 with the pinions 93is identical or similar to that of FIG. 4 or 5, a reshaped outer ringgear part 90 a is axially longer than the inner support part 91. Thering gear part 90 a has no outer peripheral gear but an inner peripheralgear, which meshes with the outer peripheral gear of the support part 91and a pinion 77 a, as shown in FIG. 7. A tip of a pinion shaft on whichthe pinion 77 a is provided (integrally formed) is disposed in parallelto the axles 35 and laterally sidewise from the support part 91 so thatthe pinion 77 a meshes with the inner peripheral gear of the ring gearpart 90 a so as to transmit output torque of the hydraulic motor 36 tothe ring gear part 90 a. The transaxle 1 needs to be modifiedcorresponding to this side-by-side arrangement of the differential bullgear 78 and pinion 77 a. The suitably modified transaxle 1 may have areduced distance between the motor shaft 74 and the differential bullgear 78 so its dimensions perpendicular to axles 35 may be minimized.

A pair of left and right retaining plates 94 a are fixed to an innerwall of the housing 33 or the like and contact the right and leftsurfaces of the outer ring gear part 90. The inner support part 91 isaxially restricted when it is supported on the axles 35, and the leftand right retaining plates 94 a prevent the outer ring gear part 90 fromaxially moving relative to the inner support part 91.

A fourth embodiment of the differential bull gear 78 shown in FIGS. 8and 9 will be explained. The ring gear part 90 has an outer peripheralgear to mesh with the pinion 77, and an inner peripheral gear meshingwith an outer peripheral gear of the support part 91. As shown in FIG.8, the ring gear part 90 is divisible into a plurality of arcuate pieces(in this embodiment, four equal pieces) 90 b in its peripheraldirection. The dividable ring gear part 90, while being made ofexpensive high-strength material, is advantageous in maintenance andcost-saving because only the damaged piece 90 b can be exchanged orrepaired. The support part 91 supports the pinions 93 and is supportedon the axles 35 similar to the above description.

As shown in FIG. 9, from the right surface of the support part 91 isextended a retaining portion 91 d along the right surface of the ringgear part 90, thereby preventing the ring gear part 90 (i.e., all of thepieces 90 b) from axial rightward movement relative to the support part91. The retaining portion 91 d is toothed to mesh with the right end ofthe outer peripheral gear of the ring gear part 90, thereby furthersurely retaining the ring gear part 90.

Screws 95 with washers 96 are screwed into the support part 91 so thatheads of the screws 95 and the washers 96 are disposed on the leftsurface of the support part 91. The screws 95 with the washers 96 are asmany as the divisional pieces 90 b so that each of the washers 96contacting the left surface of the support part 91 also contacts theleft surface of each piece 91 b so as to prevent each piece 91 b fromaxial leftward movement.

Accordingly, the washers 96 and the retaining portion 91 d of thesupport part 91 prevent all of the pieces 90 b of the ring gear part 90from axial movement relative to the support part 91. The cross-sectionof the retaining portion 91 d of the support part 91 and the screws 95with the washers 96 in this embodiment are shown for convenience. It isonly important that the washer 96 and the retaining portion 91 d areaxially opposed with the bull gear 78 therebetween.

A fifth embodiment of the different bull gear 78 shown in FIGS. 10 and11 will be explained. The support part 91 in this embodiment is radiallyextended so that the major radius (the distance from the axis of theaxles 35) of the support part 91 is longer than the major radius of thering gear part 90. The ring gear part 90 is divisible into a pluralityof pieces 90 c, similar to that of the fourth embodiment shown in FIGS.8 and 9. The support part 91 has a main portion supporting the pinions93 and supported on the axles 35. The support part 91 is provided on oneside surface thereof with an annular recess 91 e surrounding the mainportion. The ring gear part 90 is (the pieces 90 c are) inserted at oneaxial end thereof into the annular recess 91 e. The support part 91 hasan outer peripheral portion 91 f axially extended so as to entirelycover the outer peripheral surface of the ring gear part 90, as shown inFIG. 11. The axial end surface of the ring gear part 90 (the pieces 90c) located opposite to the recess 91 e is leveled with the extendedaxial end surface of the outer peripheral portion 91 f.

In the support part 91, a step between the main portion and the recess91 e and a step between the recess 91 e and the outer peripheral portion91 f are toothed to mesh with inner and outer peripheral gears of thering gear part 90 (pieces 90 c), respectively. The axial end sidesurface of the support part 91 in the recess 91 e restricts furtheraxial movement of the ring gear part 90 toward the support part 91. Theinner peripheral gear teeth of the ring gear part 90 (pieces 90 c) alsomesh with the pinion 77 a similar to that of the third embodiment shownin FIG. 7.

Screws 95 with washers 96 are axially screwed into the outer peripheralportion 91 f of the support part 91. The screws 95 with the washers 96are as many as the divisional pieces 90 c so that each of the washers 96contacts both the leveled axial end surfaces of the outer peripheralportion 91 f and each piece 90 c so as to prevent each piece 90 c fromaxial movement apart from the support part 91.

Accordingly, the support part 91 and the washers 96 prevent all of thepieces 90 c of the ring gear part 90 from axial movement relative to thesupport part 91.

A sixth embodiment of the differential bull gear 78 shown in FIG. 12will be explained. This differential bull gear 78 provided with twopinions 93 is similar to that of the first embodiment except for thereshaped support part 91. The support part 91, which is not a circulardisc, consists of only the minimum necessary portions for beingsupported on the axles 35, supporting the pinions 93, and being joinedwith the ring gear part 90, thereby being lightweight and saving costs.

Opposite toothed narrow edges 91 g of the support part 91, which aresymmetric with respect to the axial center hole 91 a, mesh with theinner peripheral gear of the ring gear part 90. The center portion ofthe support part 91 having the axial center hole 91 a is extremelynarrowed. The portion between the axial center hole 91 a and eachtoothed edge 91 g is considerably expanded so as to have the hole 91 bfor supporting the pinion 93. Corresponding to the number or arrangementof the pinions 93 or for another reason, the edges 91 g may beincreased, decreased or changed in location. Preferably, the narrowedsupport part 91 is so elastic as to cause centrifugal biasing forcetoward the ring gear part 90 for keeping the circular shape of the ringgear part 90 when the edges 91 g mesh with the inner peripheral gear ofthe ring gear part 90.

Next, a seventh embodiment of the differential bull gear 78 shown inFIGS. 13 and 14 will be explained. The illustrated ring gear part 90 isdivisible into pieces 90 b similar to that of FIG. 8. The support part91 is axially divisible into right and left plates 91 h. The right andleft plates 91 h are fastened together by bolts 97 so as to constitutethe support part 91. The support part 91 supports the pinions 93 and issupported on the axles 35, similar to the above description.

The support part 91 has an outer peripheral gear to mesh with the innerperipheral gear of the ring gear part 90. Similar to the retainingportion 91 d of the support part 91 shown in FIG. 9, each plate 91 h hasa retaining portion 91 i extended from its axial outer end surface alongthe corresponding axial outer surface of the ring gear part 90. Theretaining portion 91 i is toothed to mesh with the outer peripheral gearof the ring gear part 90. By joining the plates 91 h for making thesupport part 91, the opposite retaining portions 91 i of the plates 91 hform an annular hole therebetween so as to incorporate the ring gearpart 90. The pinion 77 is inserted into a gap between outer peripheralsurfaces of the retaining portions 91 i of the joined plates 91 h so asto mesh with the outer peripheral gear of the ring gear part 90.

For coupling the ring gear part 90 and the support part 91, the ringgear part 90 is engaged into a recess formed by the retaining portion 91i of one plate 91 h. Then, the other plate 91 h is joined to the plate91 h engaging with the ring gear part 90, whereby the two retainingportions 91 i fittingly cover the opposite axial end surfaces of thering gear part 90 so as to prevent the ring gear part 90 from axialmovement.

Incidentally, the dividable ring gear part 90 according to each of thethird to seventh embodiments may alternatively be a single undividablemember.

The concept underlying the differential bull gear 78 constituted by thejoined ring gear part 90 and the support part 91 is to make a memberfrom inexpensive material while keeping high strength in its frictional(contacting) portion. The same concept is applicable for making HSTmembers having sliding portions, such as a center section, pistons and amovable swash plate. In this regard, as for the HST 25, the centersection 38 shown in FIG. 15 and the movable swash plate 45 shown in FIG.16 will be explained.

As mentioned above, the center section 38 has the surfaces 38 a and 38b, onto which the cylinder block 40 of the hydraulic motor 36 and thecylinder block 44 of the hydraulic pump 37 are slidably rotatablymounted. While the center section 38 is economically made fromlow-density sintered powder metal or the like, the surfaces 38 a and 38b are coated with material having a low coefficient of friction and highabrasive resistance so as to enhance durability of the center section 38and ensure smooth sliding of the cylinder blocks 40 and 44 on thesurfaces 38 a and 38 b. The center section 38, if it is made fromsintered powder metal, is so porous as to cause insufficient lubricationon the surfaces 38 a and 38 b. By coating the surfaces 38 a and 38 b,the pores opening on the surfaces 38 a and 38 b are filled up so as toensure sufficient lubrication thereon. For example, ceramic material andPVD (physical vapor deposition) can be used as a material and method forcoating the surfaces 38 a and 38 b.

While the movable swash plate 45, pistons 39 and 43, and cylinder blocks40 and 44 are made of economic material, the slide surface 45 a of themovable swash plate 45 shown in FIG. 16 may be coated with suitablematerial as mentioned above, and the pistons 39 and 43 and the cylinderblocks 40 and 44 may be coated on their surfaces similarly.

Next, some embodiments of the breather 101 will be explained withreference to FIGS. 17 to 23. A vertical hole 33 c penetrates an uprightportion 33 d of the upper housing member 33 a of the housing 33 justabove the oil tank 100 shown in FIG. 3. Referring to a breather 101shown in FIG. 17, an elastic valve housing 103 is inserted downward intothe hole 33 c so as to be elastically filling the hole 33 c. A cover 104is placed on the top of the valve housing 103 upwardly projecting fromthe upright portion 33 d and surrounding the upright portion 33 d with ahorizontal annular gap between the vertical outer side surface of theupright portion 33 d and the bottom edge of the cover 104. Therefore,the cover 104 covering the valve housing 103 and the upright portion 33d of the upper housing member 33 a is open downward to the open air soas to prevent dust from entering the oil tank 100.

The valve housing 103 is formed with a top flange portion 103 h. In thevalve housing 103 is bored a downwardly open vertical air passage 103 ain which a ball valve 106 is vertically movably disposed. The top flangeportion 103 h of the valve housing 103 is partly cut away so as to forma substantially horizontal and upwardly open air passage 103 b betweenthe cover 104 and the valve housing 103. The air passage 103 b isconnected to the air passage 103 a through an upper orifice 103 f,covered at its open top with the cover 104, and open sideward toward thedownwardly open space in the cover 104.

The top of the cover 104 is so high apart from a top surface of theupright portion 33 d so as to ensure an air passage in the cover 104between the sideward opening of the air passage 103 b and the downwardopening of the cover 104. A portion of the cover 104 opposite to thesideward opening of the air passage 103 b is downwardly stepped so as toengage with the flange portion 103 h of the valve housing 103 and abutagainst the top surface of the upright portion 33 d, thereby firmlyengaging with the valve housing 103 and the upper housing member 33 aand preventing air leakage.

The valve housing 103 and the cover 104 are made of elastic material,such as rubber or synthetic resin, so that the valve housing 103 istightly fitted to the upper housing member 33 a in the hole 33 c, andthe cover 104 is tightly fitted to the valve housing 103 and the upperhousing member 33 a, thereby sealing oil in the oil tank 100 andpreventing air leakage. A vertically cylindrical reinforcing member 105made of metal or the like is buried in the valve housing 103 surroundingthe air passage 103 a so as to compensate for softness of the valvehousing 103. The ball valve 106 may be made of either hard material suchas steel or soft material such as rubber corresponding to differentsituations. The ball valve 106, if made of soft material, may be furthertightly fitted on each of the valve seats 103 c and 103 d, so that,particularly when the upper housing member 33 a is vertically reversed,the ball valve 106 fitted on the valve seat 103 c surely shut out theorifice 103 f from the valve chamber so as to enhance the certainty ofprevention of oil leakage.

The vertically middle portion of the air passage 103 a is diametricallylarger than the upper and lower portions thereof, thereby forming anupper step 103 c between the upper and middle portions, and a lower step103 d between the lower and middle portions. The vertically middleportion of the air passage 103 a is provided as a valve chamber in whichthe ball valve 106 is vertically movably disposed. The upper and lowersteps 103 c and 103 d serve as valve seats for the ball valve 106. Thelower step 103 d is partly notched to form a lower orifice 103 e openinginto the housing 33.

When the housing 33 is normally arranged in its vertical direction, theball valve 106 abuts against the lower step 103 d so that the valvechamber in the valve housing 103 communicates with the air above oillevel of the oil tank 100 in the housing 33 through only the orifice 103e, thereby breathing the air from the housing 33. If the air pressure inthe housing 33 excessively arises, the ball valve 106 is pushed up apartfrom the step 103 d so as to increase the outward airflow.

If the housing 33 is vertically reversed for some reason, such asarranging parts into the housing 33, the ball valve 106 falls to abutagainst the step 103 c so as to completely block the valve chamber fromthe orifice 103 f below the valve chamber, thereby preventing oil fromleaking out from the housing 33. The breather 101 prevents oil leakagein assembly facilities and during maintenance of the transaxle 1.

FIGS. 18 to 23 illustrate various modified breathers 101. Each of themodified breathers 101 has some common elements, i.e., the air passage103 a serving as a valve chamber, the valve 106 in the valve chamber,the upper and lower valve seats 103 c and 103 d with the valve 106therebetween, and the upper and lower orifices 103 f and 103 e with theupper and lower valve seats 103 c and 103 d therebetween. The commonimportant point ensured by all the modified breathers 101 is that, whileunder normal conditions, the lower orifice 103 e is constantly open intothe housing 33, the upper orifice 103 f is constantly open to the openair, and the valve 106 is fitted onto the lower valve seat 103 d openingthe orifice 103 e to the valve chamber, but when housing 33 is flippedvertically, valve 106 becomes fitted to the valve seat 103 c tocompletely shut the orifice 103 f from the valve chamber therebypreventing oil in the housing 33 from leaking out through the breather101.

A breather 101 shown in FIG. 18 has a plastic valve housing 103 havingrequired rigidity and strength without the reinforcing member 105.Similar to the valve housing 103 of FIG. 17, the valve housing 103 ofFIG. 18 is provided with the vertical air passage 103 a opening into thehousing 33, the sideward air passage 103 b opening to the open air, theorifice 103 f connecting the air passages 103 a and 103 b, the ballvalve 106 in the air passage 103 a, and the top flange portion 103 h.

A cup-like elastic seal member 107 made of elastic material such asrubber or synthetic resin is tightly filled in the verticallycylindrical gap between the upright portion 33 d and the valve housing103 so as to seal the gap. The seal member 107 may be bonded to thevalve housing 103 so as to be integrated with the valve housing 103. Thehorizontal bottom portion of the seal member 107 is fitted to the bottomsurface of the valve housing 103. The center bottom portion of the sealmember 107 is vertically bored throughout so as to form the lower step(lower valve seat) 103 d and orifice 103 e at the bottom opening of theair passage 103 a. The cover 104 made of elastic material is similar tothat of FIG. 17, and tightly fitted to the valve housing 103 and theupper housing member 33 a, similarly.

A breather 101 of FIG. 19 is modification of the breather 101 of FIG.18, wherein the elastic cover 104 is further integrally formed with anexpanding portion functioning similar to the seal member 107 shown inFIG. 18. That is, the expanding portion of the cover 104 is verticallycylindrical so as to be tightly filled in the vertical gap between thevalve housing 103 and the upright portion 33 d. A bottom edge of theexpanding portion of the cover 104 is partly extended horizontally so asto prevent the ball valve 106 from falling out from the valve housing103. This extended bottom edge of the cover 104 also serves as the lowervalve seat 103 d. The valve chamber of the air passage 103 a is widelyopen downward into the housing 33, however, the ball valve 106 isnormally placed on the valve seat 103 d while ensuring a small gapbetween the ball valve 106 and the open bottom peripheral edge of theair passage 103 a. This small gap serves as the lower orifice 103 e.

Each of breathers 101 shown in FIGS. 20 to 23 uses elastic valvehousings 103 in which the reinforcing member 105 is buried, similar tothat of FIG. 17. However, instead of the elastic cover 14, each of theelastic valve housings 103 is formed with a flange portion 103 ihorizontally extended from its upper portion above the upright portion33 d. An outer peripheral end of the flange portion 103 i is extendeddownward so as to elastically clamp the upright portion 103 d with amain central body of the valve housing 103 in the hole 33 c, whereby thevalve housing 103 firmly engages with the upper housing member 33 a.

The valve housing 103 used for both the breathers 101 shown in FIGS. 20and 21 is downwardly recessed at its top. A plastic lid 108 is filled inthe top recess of the valve housing 103. A top portion of the lid 108shown in FIG. 20 is centrifugally horizontally extended over the topperipheral edge of the valve housing 103 surrounding the top recess andextended downwardly at its outer peripheral end so as to clamp the topperipheral edge of the valve housing 103 with the main body of the lid108 filled in the top recess of the valve housing 103. Alternatively,the lid 108 shown in FIG. 21, fully fitted in the valve housing 103, isleveled at its top surface with the top surface of the valve housing103.

Each of the valve housings 103 of FIGS. 20 and 21 is formed therein withthe vertical air passage 103 a with the upper and lower orifices 103 fand 103 e, similar to the foregoing embodiments. The valve housing 103is further formed with a groove 103 g extended outward from the upperorifice 103 f. A horizontal part of the groove 103 g directly connectedto the orifice 103 f is open upward to the top recess of the valvehousing 103 and covered at the top opening with the lid 108 filled inthe recess. The groove 103 g is downwardly stepped from the upwardlyopen horizontal part thereof and downwardly open at the inner surface ofthe bent flange portion 103 i of the valve housing 103. A downwardlyoutward opening of the groove 103 g is disposed at the bottom edge ofthe bent flange portion 103 i along the outer peripheral surface of theupright portion 33 d.

Each of the breathers 101 of FIGS. 22 and 23 is provided with anupwardly exposed valve housing 103 without a covering member such as thecover 104 or the lid 108. The valve housing 103 is formed with adownwardly outward open vertical groove 103 j on the inner surface ofthe vertical peripheral edge of the flange portion 103 i along theupright portion 33 d. The vertical groove 103 j is inwardly open to aspace between the horizontal bottom surface of the flange portion 103 iand the top surface of the upright portion 33 d.

The valve housing 103 shown in FIG. 22 is bored therein with a slantingsideward air passage 103 k connected to the top end of the vertical airpassage 103 a through the upper orifice 103 f. The air passage 103 k isopen to the space between the flange portion 103 i and the uprightportion 33 d, thereby communicating with the outwardly open verticalgroove 103 j.

The valve housing 103 shown in FIG. 23 is bored therein with ahorizontal air passage 103 m directly connected to the top end of thevertical air passage 103 a. A plug 110 plugs an outward opening of thehorizontal air passage 103 m. The valve housing 103 is bored thereinwith the upper orifice 103 f, which is vertically downwardly extendedfrom an intermediate portion of the horizontal air passage 103 m andopen at its bottom end to the space between the flange portion 103 i andthe upright portion 33 d so as to communicate with the outwardly openvertical groove 103 j.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes within the purviewof the appended claims may be made without departing from the true scopeand spirit of the invention in its broader aspects.

1. A bull gear of a differential gear assembly, comprising: a ring gearfor inputting torque from a transmission; and a support membersupporting a differential pinion and supported on an axle, the supportmember transmitting the torque from the ring gear to the axle throughthe differential pinion, wherein the ring gear and the support memberare coupled together so as to be relatively axially shiftable andnot-relatively rotatable; wherein the ring gear has a toothed outerperiphery and a toothed inner periphery, wherein either the toothedouter or inner periphery serves as an input gear for receiving torquefrom the transmission, and wherein both of the toothed outer and innerperipheries mesh with a toothed portion of the support member so as tocouple the ring gear with the support member.
 2. The bull gear of adifferential gear assembly as set forth in claim 1, the support memberhaving an annular recess into which the ring gear is fitted, wherein therecess has a toothed outer periphery and a toothed inner periphery formeshing with the toothed outer and inner peripheries of the ring gear,respectively.
 3. The bull gear of a differential gear assembly as setforth in claim 2, the support member being divisible into halves each ofwhich has the annular recess, wherein the halves are joined to eachother so as to fit the ring gear in the mutually facing recesses,thereby forming the support member holding the ring gear.
 4. The bullgear of a differential gear assembly as set forth in claim 1, the ringgear having a first surface perpendicular to the axle, and the supportmember having a second surface to be leveled with the first surface ofthe ring gear, further comprising: a retaining member abutting againstthe first and second surfaces so as to prevent the ring gear and thesupport member from relatively axially moving; and a screw screwed intoeither the ring gear or the support member, wherein the retaining memberis a washer provided on the screw.
 5. The bull gear of a differentialgear assembly as set forth in claim 1, wherein the ring gear has a firstsurface perpendicular to the axle, and the support member has a secondsurface level with the first surface of the ring gear, and wherein thering gear is divided into a plurality of pieces, further comprising: aplurality of retaining members abutting against the first and secondsurfaces so as to prevent the ring gear and the support member fromrelatively axially moving.